Process for metal surface conditioning



United States Patent Office 3,489,625 PROCESS FOR METAL SURFACE CONDITIONING Gaillard W. Dell, Detroit, and Edwin W. Goodspeed,

Union Lake, Mich., assignors to Hooker Chemical Corigoration, Niagara Falls, N.Y., a corporation of New ork No Drawing. Continuation-impart of application Ser. No. 456,539, May 17, 1965. This application June 14, 1968, Ser. No. 736,969

Int. Cl. C23b 3/00 US. Cl. 156-18 7 Claims ABSTRACT OF THE DISCLOSURE A process for pickling carbon steel having a passivated surface film which comprises contacting the passivated surface with an acid pickling solution for at least minutes, contacting the pickled surface with an alkaline permanganate solution and then again contacting the surface with an acid pickling solution until a matte finish is formed on the surface.

This application is a continuation-in-part of copending application Ser. No. 456,539, filed May 17, 1965, now abandoned.

This invention relates to an improved method for treating metal surfaces and more particularly it relates to an improved method and composition for treating ferrous metal surfaces to produce a dull or matte finish thereon.

In the preparation of metal surfaces, and particularly ferrous metal surfaces, to receive a coating, such as an electro coating; a protective or paint base coating, such as a phosphate coating; and/ or a paint coating or the like, it is generally desirable that the metal surface have a dull or matte finish. When coatings of this type are applied over such a dull or matte finish, there is generally much better adhesion between the coating and the surface than where the surface has a more glossy or lustrous finish. Normally, this desirable dull or matte finish is obtained by subjecting the ferrous metal surface to the action of an acid pickling solution, such as an aqueous solution of sulfuric acid or hydrochloric acid.

In some instances, however, it has been found that carbon steel surfaces to be treated have a passivated film which is substantially unaffected by conventional treatment with acid pickling solutions. In such cases, treatment with an acid pickling solution does not produce a dull or matte finish on the ferrous metal surface, but a lustrous, or at least semi-lustrous, surface remains after the treatment. It has been found that where this passivated film remains on the ferrous metal, difiiculties may be encountered in obtaining high quality, adherent coatings on the surface, such as those indicated above.

In some instances, it has been found that this passivated surface on the carbon steel may be physically removed, as for exam le by grinding or sanding, but such techniques are generally not desirable. Not only is a grinding or sanding operation costly and time consuming, particularly where irregular surfaces are involved, but, additionally, if the dimensions of the part are changed by such operations, use of these techniques would probably be precluded. Accordingly, up to the present time, there has not been a completely satisfactory method for easily and economically treating carbon steel which has a passivated surface, in order to produce a dull or matte finish on the metal.

It is, therefore, an object of the present invention to provide an economical and simple method for treating carbon steel having a passivated surface so as to produce a matte finish on the metal.

3,489,625 Patented Jan. 13, 1970 A further object of the present invention is to provide an improved process for treating carbon steel having a passivated surface, so as to produce a matte finish on the metal, which process does not produce any substantial dimensional change in the metal being treated.

These and other objects will become apparent to those skilled in the art from the description of the invention which follows.

Pursuant to the above objects, the present invention includes a process for treating carbon steel having a passivated surface which is normally substantially unaffected by an acid pickle, which process comprises contacting the passivated surface of the steel with an acid pickling solution, maintaining the solution in contact with the surface for a period of at least 5 minutes but insufficient to form a carbonaceous smut on the surface, contacting the surface with an alkaline permanganate solution, and, thereafter, contacting the surface With an acid pickling solution for a period of time sufficient to produce a matte finish on the surface. In the operation of this process, the combination of the acid pickling solution followed by the alkaline permanganate solution in some manner attacks the passivated surface of the carbon steel, thus making the subsequent acid pickling solution effective in producing a matte finish on the surface. Neither the acid pickle nor the permanganate solution, alone has been found to produce these results.

More specifically, in the practice of the method of the present invention, carbon steel having a passivated surface which is normally substantially unaffected by an acid pickling solution is contacted with an aqueous acid pickling solution. Although it is preferred to effect the contact with the acidic pickle by immersing the metal in the acidic pickling solution, it is to be appreciated that te pickling solution may be brought into contact with the passivated surface in other ways, as for example, by spraying, flowing, or the like. In many instances, however, it has been found that the most economical usage of the acidic pickling solution is obtained when the metal is immersed therein and, for this reason, primary reference Will be made hereinafter to contacting the metal with the aqueous acidic pickling solution by immersion.

The passivated surface is maintained in contact with the aqueous acidic pickling solution for a period of at least 5 minutes. Generally, contact times within the range of about 5 to about 45 minutes are typical, with contact times within the range of about 8 to about 20 minutes being preferred. Although in some instances, longer contact times may be used, it is important that the length of the contact between the pickling solution and the passivated surface is not sufficient to effect the formation of a carbonaceous smut on the surface. According, in selecting the contact time, this factor should also be kept in mind so that the usable times for contact between the acid pickling solution and the carbon steel are within the range of 5 minutes up to a time which will produce a substantial amount of a carbonaceous smut on the surface of the steel.

Various aqueous acid pickling solutions may be used, as are known to those in the art. These may be formulated from various acids including organic acids, and mineral acids such as sulfuric acid, hydrochloric acid, phosphoric acid, and the like. Typically, the pickling solutions are aqueous solutions of mineral acids, and are preferably aqueous solutions of sulfuric acid or hydrochloric acid. Although various concentrations of these acid solutions may be used, concentrations within the range of about 5 to about 25 percent being typical, acid concentrations within the range of about 10 to about 20 percent by Weight of. the solution are preferred.

After the contact with the aqueous acid pickling solution for the desired time, the passivated surface is then contacted with an alkaline permanganate solution. Here again, the contact is preferably effected by immersing the passivace surface in the permanganate solution, although other contacting techniques may also be used. The alkaline permanganate solution is an aqueous alkaline solution containing an alkali metal permanganate. By alkali metal permanganate, it is intended to refer to the permanganates of lithium, sodium, potassium, cesium and rubidium. Of these, the preferred is potassium permanganate and, accordingly, primary reference will be made hereinafter to this material. The amount of potassium permanganate in this solution will be at least that amount which will provide the desired activation of the surface to make the subsequently applied acidic pickling solution effective in forming a matte finish on the metal. Typically, the potassium permanganate is present in the treating solution in an amount within the range of about 5 t about 50 grams per liter and preferably in an amount within the range of about 15 to about 35 grams per liter. It is to be appreciated, of course, that in some instances, the treating solution may contain potassium permanganate in amounts either greater than or lesser than those which have been indicated, provided that the amounts used are sufficient to provide the above indicated desired activation of the passivated metal surface.

In addition to the alkali metal permanganate, the metal conditioning solution of the present invention also contains an alkali metal hydroxide. Although various alkali metal hydroxides may be used, such as the hydroxides of lithium, sodium, potassium, cesium or rubidium, it has generally been found to be preferable to used sodium hydroxide in the present treating solutions. Typically, the sodium hydroxide is present in an amount within the range of about to about 200 grams per liter, with an amount within the range of about 20 to about 150 grams per liter being preferred. It is believed that the alkalinity added to the treating solution by the addition of the sodium hydroxide in some manner increases the effectiveness of the permanganate in activating the passivated surface so as to make the subsequently applied acidic pickling solution effective in producing a matte finish on the surface. Accordingly, in some instances both greater and lesser amounts of the alkali metal hydroxide may be used in the treating solution, provided that whatever amount is used is at least sufficient to obtain this desired enhancement of the permanganate solutions.

The alkaline permanganate solution is maintained in contact with the passivated surface for a period of time which is at least sufficient to effect the desired activation of the surface so that the subsequently applied acidic pickling solution will be effective in forming a matte finish on the surface. Typically, the contact times used are within the range of about 2 to about 20 minutes, with contact times within the range of about 5 to about minutes being preferred. As with the acid pickling solution, both shorter and longer contact times may be used in some instances, provided the requisite activation of the surface is obtained.

After the desired contact of the passivated surface with the potassium permanganate solution, the surface is then again brought into contact with an acidic pickling solution. The pickling solutions which may be used are the aqueous acidic solutions as have been described hereinabove and the passivated surface may be brought into contact with them by immersion or other contacting techniques as have been heretofore described. Generally, the acidic pickling solution with which the surface is brought into contact after treatment with the permanganate solution is of the same type as the acid pickling solution with which the surface is contacted prior to treatment with the alkae line permanganate solution. It is to be appreciated, however, that if desired, a different acid pickling solution may be used in the treatments which pres tl a d Q QW th alkaline permanganate treatment. After the permanganate treating step, the passivated steel surfaces are maintained in the acid pickling solution for a period of time which is suiiicient to produce a dull or matte finish on the metal. Here again, it is important that the contact times are not suflicient to produce a deposit of carbonaceous smut on the metal surface. Exemplary of contact times which are used are those up to about 5 minutes, with times Within the range of several seconds up to about 3 minutes being typical.

After the second acidic pickling step, the carbon steel surfaces which have been treated are then ready for the application of various coatings, such as electroplating, phosphate coatings, paint, and the like. If it is desired, the surfaces may be rinsed with Water after they are removed from the second acid pickling solution so as t remove any residual acid from the metal surface. Similarly, water rinses may be used between the other treating steps of the process, i.e., between the first acidic pickling solution and the permanganate solution and between the permanganate solution and the second acidic pickling solution.

In formulating the alkaline permanganate solutions for use in the present process, a dry composition is formulated by admixing the potassium permanganate and sodium hydroxide, in amounts sufficient to provide the desired concentration of these components in the aqueous solution. This dry composition is then admixed with water to form the aqueous treating solution. Typically, such a dry composition may contain from about 40 to about parts by Weight of sodium hydroxide and from about 5 to about 60 parts by weight of potassium permanganate. Amounts of sodium hydroxide within the range of about 60 to about 90 parts by weight and amounts of potassium permanganate within the range of about 10 to about 40 parts by weight are preferred.

In formulating such a dry composition, it may also be desirable to include therein an alkali metal carbonate, such as sodium carbonate. Typically, where the sodium carbonate is included in the dry mixture, it is present in an amount within the range of about 5 to about 50 parts by weight being preferred. A specifically preferred com position of this type, for use in formulating the aqueous alkaline permanganate treating solution of the present invention, is one containing about 60 parts by weight of sodium hydroxide and about 20-25 parts by weight of potassium permanganate and about 15-20 parts by weight of sodium carbonate.

As has been noted hereinabove, such a dry mixture is combined with water in amounts sufficient to give an aqueous treating solution containing the alkali metal hydroxide and alkali metal permanganate in the amounts which have been indicated. Where the dry mixture also contains an alkali metal carbonate, this component is present in the aqueous treating solution in an amount within the range of about 5 to about 50 grams per liter and preferably in an amount within the range of about 15 to about 35 grams per liter. A specifically preferred aqueous treating solution is one containing about grams per liter of sodium hydroxide and about 30 grams per liter each of potassium permanganate and sodium carbonate.

In the practice of the method of the present invention,

carbon steel having a passivated surface which is normally substantially unaffected by an acid pickling solution is contacted, preferably by immersion, with an aqueous acidic solution, such as an aqueous solution of sulfuric or hydrochloric acid. This acidic pickling solution is maintained in contact with the passivated metal surface for a period of about 15 minutes. The metal surface is then removed from the pickling acid and, preferably, rinsed with Water so as to remove any of the acid solution which remains on the surface. Thereafter, the passivated metal surface is contacted, preferably by immersion, with an aqu-. eous alkaline permanganate solution, preferably containing o t LZQ g m per l te of o ium y exid 3 g ms.

per liter of potassium permanganate and 20 grams per liter of sodium carbonate. Desirably, this solution is at an elevated temperature above room temperature, i.e., 20 degrees eentigrade, temperatures within the range of about 25 to about 100 degrees centigrade being typical with temperatures within the range of about 50 to about 85 degrees centigrade being preferred. The metal surface is preferably maintained in contact with the permanganate solution for a period of about minutes. The passivated metal surface is then desirably rinsed with water so as to remove any of the alkaline permanganate solution remaining on the surface and is then contacted again with the aqueous acidic pickling solution. Preferably, this solution is the same as the one which has previously been used and the surface is maintained therein for a period of time suflicient to produce a matte finish on the metal. For this purpose, contact times of about 2 minutes have often been found to be suitable. When the matte finish has been produced on the surface of the carbon steel, it is removed from the acidic solution and, desirably, again rinsed with water to remove any acid which remains on the surface. The surfaces thustreated are then ready for the application of any subsequent coating, as has been indicated hereinabove.

It is to be appreciated that in referring to carbon steel it is intended to mean those steels having a carbon content of from about 0.081.03% and having an A.I.S.I. designation in the C 1000 series. Additionally, although the exact nature of the passivated film on the surface of the carbon steel is not known, this passivated film is believed to be substantially pure ferrite, formed as a result of the steel being subjected to reducing conditions during previous processing. The film is light or whitish in color, lustrous and substantially uneffected by acid pickling. Thus, as has been previously noted, after conventional acid pickling of such passivated film, a dull, matte finish is not formed on the surface but it retains its light colored, lustrous or semi-lustrous finish.

In order that those skilled in the art may better understand the present invention and the manner in which it may be practiced, the following specific examples are given It is to be understood that these examples are given for illustrative purposes only and it is not intended that they will limit the scope of the invention. In these examples, unless otherwise indicated, temperatures are in degrees centigrade and parts are by weight.

EXAMPLE 1 Slugs of 1020 steel were immersed in an aqueous solution of sulfuric acid containing about percent by weight of H 80 for a period of about 15 minutes. The sulfuric acid solution was at a temperature of about 75 degrees centigrade. These slugs, prior to immersion in the sulfuric acid solution had a whitish, lustrous, passivated film on the surface. After removal from the sulfuric acid solution and rinsing in water to remove any remaining acid from the surface of the slugs, it was noted that the lustrous, passivated film on the surface was substantially unchanged by the contact with the acid. These slugs were then immersed for 10 minutes in an aqueous solution containing about 108 grams per liter of sodium hydroxide and about 36 grams per liter each of potassium permanganate and sodium carbonate. This aqueous solution was at a temperature of about 80 degrees centigrade and was prepared by dissolving in water a dry composition containing about 60 parts by weight of sodium hydroxide and about parts by weight each of potassium permanganate and sodium carbonate, at a rate to provide a total dry mix concentration of about 180 grams per liter. After 10 minutes the slugs were removed from the permanganate solution and it was noted that the lustrous, passivated film on the metal surface was still substantially unchanged. The slugs were then rinsed in water and immersed in the acid solution a second time for a period of 2 minutes. At the end of this time, the slugs were removed from the acid solution and were found to have a dull, matte finish on the surface, the lustrous, passivated film, which had previously been on the surface, having been completely removed. Upon treating these slugs with a conventional zinc phosphate coating solution, an excellent, adherent, protective phosphate coating was produced on the slugs.

EXAMPLE 2 The procedure of Example 1 was repeated with the exception that the aqueous alkaline permanganate solution contained only 30 grams per liter of potassium permanganate and 20 grams per liter of sodium hydroxide. This solution was formulated by dissolving in water a dry composition containing about 60 parts by weight of potassium permanganate and 40 parts by weight of sodium hydroxide. As in the previous example, after the second aqueous acidic solution treatment, there was produced on the surface of the slugs a dull, matte finish.

EXAMPLE 3 The procedure of Example 1 was repeated with the ex ception that the aqueous permanganate solution contained only 15 grams per liter of potassium permanganate and 20 grams per liter of sodium hydroxide. This solution was formulated by dissolving in water a dry composition containing about 57 parts by weight of sodium hydroxide and 43 parts by weight of potassium permanganate. As in the previous examples, the lustrous, passivated film on the surface of the steel slugs was completely removed in the second acid pickling solution and there was obtained on these slugs a dull, matte finish.

EXAMPLE 4 By way of comparison, steel slugs of the same type used in Example 1 were immersed in an acid pickling solution as described in Example 1 for a period of 45 minutes. At the end of this time the slugs were removed and it was found that the lustrous, passivated film on the surface was substantially unalfected by the acid treatment. Upon treating these slugs with a conventional zinc phosphate coating solution, the resulting coating formed was of poor quality, having poor adhesion and being generally unsatisfactory.

EXAMPLE 5 By way of further comparison, the procedure of the proceeding example was repeated with the exception that for the acid pickling solution, there was used only an alkaline permanganate solution as describedin Example 1. After removal from this permanganate solution, it was found that the lustrous, passivated film on the slugs was substantially unaffected by the solution. A portion of these slugs was then contacted with an acid pickling solution, as in Example 1, and maintained in this solution for a period of about 15 minutes, at which time, traces of a carbonaceous smut were beginning to form on the metal surface. Upon removing these slugs from the pickling solution, it was found that the lustrous, passivated film on the surface of the metal was substantially unaffected by the pickling solution. Upon treating both sets of slugs with a conventional acidic zinc phosphate coating solution, the coatings produced were of poor quality, having poor adhesion and being generally unacceptable.

The procedure of Example 1 was repeated using a hydrochloric acid pickling solution and a permanganate treating solution containing sodium permanganate, potassium hydroxide and potassium carbonate, and comparable results were obtained.

While there have been described various embodiments of the invention, the compositions and methods described are not intended to be understood as limiting the scope of the invention, as it is realized that changes therewithin are possible, and it is further intended that each element recited in any of the following claims is to be understood as referring to all equivalent elements for accomplishing substantially the same results in substantially the same or equivalent manner, it being intended to cover the invention broadly in whatever form its principles may be utilized.

What is claimed is:

1. A process for pickling carbon steel having a passivated surface which is normally substantially unaffected by an acid pickling solution which comprises contacting said passivated surface with a sulfuric or hydrochloric acid pickling solution for a period of at least minutes, contacting the thus-treated surface with an aqueous, alkaline permanganate solution containing from about 5 to 50 grains per liter of an alkali metal permanganate and from about to 200 grams per liter of an alkali metal hydroxide, and, thereafter, again contacting the surface with an acid pickling solution for a period sufficient to produce a matte finish on the surface.

2. The process as claimed in claim 1 wherein the aqueous alkaline permanganate solution also contains an alkali metal carbonate in an amount from about 5 to 50 grams per liter.

3. The process as claimed in claim 2 wherein the alkali metal permanganate is present in an amount from about to 35 grams per liter, the alkali metal hydroxide is present in an amount from about to 150 grams per liter and the alkali metal carbonate is present in an amount from about 15 to grams per liter.

4. The process as claimed in claim 3 wherein the alkali metal permanganate is potassium permanganate, the alkali metal hydroxide is sodium hydroxide and the alkali metal carbonate is sodium carbonate.

5. The process as claimed in claim 4 wherein the passi- -vated surface is first contacted with the acid pickling solution for a period of from about 5 to minutes and is then contacted with the alkaline permanganate solution for a period of from about 2 to 20 minutes.

6. The process as claimed in claim 5 wherein the passivated surface is first contacted with the acid pickling solution for a period of from about 8 to 20 minutes and is then contacted with the alkaline permanganate solution for a period of from about 5 to 15 minutes.

7. The process as claimed in claim 6 wherein the acid pickling solutions used are selected from aqueous solutions of hydrochloric acid and sulfuric acid.

References Cited 7 UNITED STATES PATENTS JACOB H. 'STEINBERG, Primary Examiner US. (:1. X.R. 

